The goal of this research project is two-fold: 1) to describe the cardiac action potential in terms of ionic currents and membrane conductance as precisely as possible, and 2) to determine the effects which various agents have on cardiac membrane permeability. The sucrose-gap voltage clamp technique will be applied to frog atrial bundles. By this method, the potential across the cardiac membrane can be controlled and the membrane ionic current measured as a function of membrane potential and time. The specific ions (e.g., sodium and potassium) which contribute to the membrane current will be identified from alterations in membrane current produced by extracellular solutions of various ionic composition. It will be possible by these experiments to determine the time and voltage dependent changes in cardiac membrane conductance responsible for the cardiac action potential and the ions carrying the current during the different phases of the action potential. Changes in current-voltage relationships produced by various agents (e.g., ions, acetylcholine, epinephrine, etc.) will indicate the effects which these agents have on cardiac membrane permeability.